Tidy up requirements.txt

[felix-schott]

Description

The requirements.txt/requirements_dev.txt files need some tidying up, it seems. I see leafmap in there but don't think it's used anywhere in the code. In addition, all packages should ideally be pinned to specific versions - otherwise there is a risk that pip downloads the latest available version at a given point in time which can even break versions of the package that used to work for users.